HOWARD: CARRIAGE OF DISEASE BY INSECTS 219 



Moreover in inoculation experiments reported by Flexner and Lewis, 

 the virus is present in the blood of inoculated monkeys in such high 

 dilution that the infection in a normal animal is accomplished only 

 by inoculation of 20 cc. or more of the blood. If this holds under 

 normal conditions, it becomes absurd to accuse any biting insect of the 

 carriage of this particular disease, except in the possible event of the 

 development of the organism in the body of the insect. While this 

 possibility should be studied, the probabilities are against it. The 

 impression which all of us in Doctor Flexner's audience at his lecture 

 given December 28, in New York, gained, was I think that we are still 

 greatly in the dark in regard to this disease, but that possible insect 

 carriage must probably be ruled out. 



It was then shown that it is necessary to divide the field under 

 discussion into three categories. 



1. Insects as simple carriers of disease, the accidental carriers 

 as it were; that is, insects frequenting places where disease germs 

 are likely to occur and conveying these in their stomachs or on 

 their bodies to food supplies. This is notably illustrated by the 

 house fly. 



2. Insects as direct inoculators of disease. These are biting 

 insects which feed upon diseased men or animals and carry the 

 causative organisms on then- beaks and insert them into the 

 circulation of healthy animals. In this way anthrax is carried 

 by biting flies; surra is carried in the same way, as is also the 

 nagana or tsetse-fly disease of cattle. So also is bubonic plague 

 carried in this manner by rat-fleas, but here there is more than a 

 passive carriage, as is also the case with the tsetse-fly disease. 



3. The third category, and this is perhaps the most important, 

 includes insects as essential hosts of pathogenic organisms. These 

 are the cases in which the parasitic organism undergoes its sexual 

 generation in the body of its insect host and another, non-sexual, 

 generation or generations in its warm-blooded host. To this 

 class belong the malarial mosquitoes, the yellow-fever mosquito, 

 and the rapidly increasing number of species that carry trypano- 

 miases, leishmanioses, spirochaetoses, and the ticks that carry 

 relapsing fevers and other fevers of man and animals, and the 

 lice that carry typhus fever. 



Under the first of these three categories the house fly was 

 considered at some length, and cockroaches, ants (especially 





